1. Field Of The Invention
The present invention relates to the art of continuously forming sheet material into an undulating pattern.
More particularly, the present invention relates to an improved apparatus for nip forming paper and plastic materials into a corrugated sheet.
2. Description Of The Prior Art
Corrugated paperboard is a structural product fabricated from two or more laminae of paper sheet, usually in the order of 0.009 inch (0.229 mm) thick, one of the laminae being formed to an undulating or "corrugated" pattern. Relative to one or both faces of the corrugated lamina, a "liner" sheet is adhesively secured to the corrugation crests.
The fabrication of corrugated board is carried out on machinery of a generally standardized design comprising two steam heated corrugating rolls, a pressure roll and an adhesive application roll system.
The corrugating rolls are hollow cylinders mounted for rotation about the axes thereof, the surface of said cylinders being shaped to the desired corrugation pattern.
Each period of undulation in the corrugation pattern is characterized as a "flute" and on the corrugating rolls, the flutes are disposed parallel with the roll axes.
The two corrugating rolls are disposed relative to each other whereby the respective flutes intermesh gear fashion to create a rolling nip therebetween. The heat and pressure of this nip on a paper sheet drawn therein forces a permanent set in the sheet with the corrugation pattern.
After emerging from the corrugating nip, the sheet is held tightly to the undulating surface of one corrugating roll by a number of "fingers" distributed along the roll length. The objective of these fingers is to precisely position the fluted sheet for application of adhesive to the crests thereof. Such adhesive is applied as a film wiped from the surface of a glue applicator roll as the flute crests pass tangently thereagainst.
Subsequently, around the one corrugating roll periphery, the adhesive carrying flute crests enter a nip with a smooth surface pressure roll into which is fed a liner sheet. The consequent nip pressure between the adhesive carrying flute crests of the corrugated sheet and the liner sheet bonds the two together to form a "single face" board.
Thereafter, the board is separated from the one corrugating roll surface for subsequent processing such as application of another liner sheet to flute crests on the opposite side of the corrugated sheet to form "double face" board.
The use of fingers to hold the corrugated sheet to the corrugating roll surface betwen the corrugating nip and the liner board nip constitutes the source of numerous operational and product integrity problems.
One such problem is the fact that a continuous score crease is pressed into the flute crests by internal sheet stresses which force the sheet strongly against the narrow finger edge. Such score creases ultimately become lines of weakness and failure for the board product.
Also, as a consequence of sheet stresses against the fingers, is a longitudinal bowing of the sheet flutes. For this reason, the finger objective is not entirely achieved since such bowing of the flutes represents a dimensional variation along the flute length which the fixed dimension cylindrical glue applicator roll cannot accommodate. Accordingly, no adhesive is applied to the flute crests in the proximity of the fingers and too much adhesive is applied to the center span between the fingers.
If the corrugated sheet is also coated with a polymer plastic for water proofing and other properties, it is heated to a tacky consistency for liner bonding in lieu of adhesive. In this state, the plastic wipes off onto the fingers and accumulates to the point of sheet damage.
For the foregoing reasons, corrugating roll fingers have been merely tolerated as a poor compromise to achieve necessary objectives. Although the use of vacuum to secure the corrugated sheet to the roll surface has been considered as an alternative to fingers, the concept has not received industry acceptance because of mechanical conflict with roll heating steam which is delivered to the roll interior. Prior art systems have presented the industry with a one or the other choice which has heretofore been resolved in the favor of steam heating.
Accordingly, it is the objective of the present invention to disclose and describe a corrugating system whereby the sheet may be simultaneously held to the corrugating roll surface by vacuum and also heated by flowing steam through the roll interior.
Another objective of the present invention is to teach the construction of a corrugating roll whereby a select, fixed position arc of the roll may be surface evacuated as the roll rotates about the axis thereof.
Another objective of the present invention is to teach the construction of a corrugating roll whereby a vacuum draft system is inexpensively disposed adjacent the interior periphery of the cylindrical shell and in intimate heat transfer contact therewith, the roll axis being left free for the disposition of a pressurized flow journal for steam.